1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a facsimile machine, and a copier, and an image forming method.
2. Description of the Related Art
Image forming apparatuses such as low-cost laser-beam printers employ a contact-type developing method performed with the use of a non-magnetic one-component developer. With this method, the image forming apparatus can have a simple structure and power source costs can be reduced. In the contact-type developing method performed with the use of a non-magnetic one-component developer, no gap is formed at the developing nip between a photoconductor, which acts as a latent image carrier, and toner. Therefore, in this method, a wraparound electric field is not generated, unlike the case of using a two-component developer (hereinafter, “two-component developing”) or using a one-component developer in a non-contact-type developing method (hereinafter, “one-component non-contact developing”). Accordingly, in the contact-type developing method performed with the use of a one-component developer, an edge effect does not occur as much as in the case of two-component developing or one-component non-contact developing; hence, the latent image can be precisely developed.
When light is written onto a photoconductor to form an isolated one-dot image, the latent image electric potential distribution is substantially a Gaussian distribution (normal distribution). In the case of two-component developing or one-component non-contact developing, an edge effect occurs, and therefore an isolated one-dot image can be reproduced with a weak laser beam. However, in the case of a contact-type developing method performed with the use of a non-magnetic one-component developer, a wraparound electric field is not generated, and therefore an isolated one-dot image cannot be properly reproduced with a laser beam having the same intensity as that used in two-component developing or one-component non-contact developing.
As a result, as indicated by a line joining ⋄ marks in the graph shown in FIG. 11, the low density portions in area gradation including isolated one-dot images will have a lower density than the ideal density.
In order to enhance the reproducibility of isolated one-dot images, various measures are taken, such as intensifying the laser beam or adjusting the developing bias (see, for example, Patent Document 1).    Patent Document 1: Japanese Laid-Open Patent Application No. 2002-292929
If the laser beam is intensified in an attempt to enhance the reproducibility of isolated one-dot images, the following problem arises. That is, gradation loss will occur (portions that are supposed to be blank will be developed) in high density portions of the area gradation, as indicated by a line joining ● marks in the graph shown in FIG. 11.
Area gradation is described with reference to FIG. 12. In a matrix of 4 dots×4 dots=16 dots, a first gradation level is expressed by forming an image at one portion (one dot) of the 16 dot matrix. As the gradation level increases, more portions of the 16 dot matrix will include dot images. At a sixteenth gradation level, the entire 16 dot matrix will be filled with dot images. In FIG. 12, the matrix includes a region corresponding to dot images (the black portions in the figure) and a region corresponding to non-image dots (the white portions in the figure). However, in reality, an error diffusion method is employed to disperse the image dots and the non-image dots.
In FIG. 13, (a) illustrates an example of potentials on a photoconductor surface at a low density portion in the area gradation and corresponding dot images. In FIG. 13, (b) illustrates an example of potentials on a photoconductor surface at a high density portion in the area gradation and corresponding dot images.
As shown in (a) of FIG. 13, at a low density portion in the area gradation, non-image dots are continuously arranged, and each dot image is isolated. In such a case, by intensifying the laser beam, the reproducibility of one-dot images can be enhanced so that favorable gradation properties are attained.
As shown in (b) of FIG. 13, at a high density portion in the area gradation, dot images are continuously arranged, and each non-image dot is isolated. The laser beam is intensified in an attempt to enhance reproducibility of isolated one-dot images. Thus, if each dot image on either side of the isolated non-image dot is used for exposing, the potential of the isolated non-image dot will be attenuated. As a result, each of the isolated non-image dots will have a potential (potential of exposed portions) that is lower than the developing bias, and the isolated non-image dots will be developed (i.e., portions corresponding to isolated non-image dots, which are supposed to be blank, will appear as dot images in the developed image). In this manner, gradation loss will occur in high density portions of the area gradation.
Another method of enhancing reproducibility of isolated one-dot images is to adjust the developing bias.
FIG. 14 illustrates an example in which the developing bias is adjusted to enhance the reproducibility of isolated one-dot images.
As shown in (a) of FIG. 14, in order to enhance the reproducibility of isolated one-dot images by adjusting the developing bias, the latent image region is developed with the use of the developing bias (2), which is closer to the potential of unexposed portions of the photoconductor than the conventional developing bias (developing bias (1)). Accordingly, the developed latent image region can be made to have a width of one dot, thereby enhancing the reproducibility of isolated one-dot images.
Furthermore, if the developing bias is adjusted to enhance the reproducibility of isolated one-dot images, the width of a latent image potential distribution on the surface of a photoconductor will be narrower compared to the case of intensifying laser beams. Thus, as shown in (b) of FIG. 14, at each isolated non-image dot, the latent image potentials that are adjacent to the isolated non-image dot (on opposite sides thereof) in the main scanning direction are not overlapping each other. Therefore, the potential at the non-image dot will not become as low as the potential of the exposed portions. As a result, gradation loss will not occur in high density portions of the area gradation.
However, even by adjusting the developing bias, the potential on the photoconductor surface significantly attenuates at portions where dot images are continuously arranged, although not as much as that in the case of intensifying the laser beam. In the case of adjusting the developing bias to enhance the reproducibility of isolated one-dot images, the developing bias is made to be closer to the potential of unexposed portions of the photoconductor. For this reason, the difference between the potential of exposed portions of the photoconductor surface and the developing bias (developing potential) becomes large. As a result, at portions where dot images are continuously arranged, the toner density becomes dark. Thus, there has been a problem in that the image density becomes dark at mid-density portions to high density portions in the area gradation where dot images are continuously arranged.